But it fails - looking at the evt object (hence my console line) it is, as I thought it would be, a polygon after three nodes have been drawn - output below, first is after a single click (ie still drawing), second after a double click

Ok - so two issues now - I think it is time to call the new features in 3.10 as broken, don't you? We need a few more events to hook into and the measure end event to be just that. An event that fires when the measurement has ended - it works for the length, which returns the value after each click, but only fires the measure end event after a double click.

What I noticed was a bug on the workaround suggested by jonathan. Never the less measure end event would be one of those important event, I used it in couple of my project, thankfully they are still on good old 3.3

On-measure-end is quite clear in what it should do, an event that fires when measurement is ended. It doesn't. It fires when a polygon has been created and an intermediate measurement has been calculated.

This only fails for area. For length a result is returned with each click without firing the on-measure-end event.